The invention is based on a priority application EP 03292629.7 which is hereby incorporated by reference.
The present invention relates to wireless communications systems, and more particularly, to a method for allocating subcarriers and selecting a modulation scheme per subcarrier in a high-speed wireless fixed or mobile multi-carrier system.
An approach to multi-carrier modulation is Orthogonal Frequency Division Multiplexing (OFDM). OFDM was for example standardized for the two high-speed digital radio transmission systems Digital Audio Broadcasting (DAB) and Digital Video Broadcasting Terrestrial transmission mode (DVB-T), which are used for transmitting digital radio and television signals. A further field of application, for which the OFDM transmission system plays an increasing role, is the mobile access to wire-bound networks with the aid of a local radio network, whereby high data rates shall also be transmitted. In this connection, the HIPERLAN/2 standard as well as an extension of the IEEE 802.11a standard for the 5 Ghz area con be mentioned. In both systems OFDM transmission is applied. OFDM could also offer a sensible alternative for high-speed mobile applications, and thus represents an important step for next generation mobile radio systems or for a 4th generation air interface.
In conventional multi-carrier schemes the transmitted data is split into a number of parallel data streams, each one used to modulate a separate subcarrier. If an OFDM transmission system is applied, the broadband radio channel is subdivided into a plurality of narrow-band subchannels or subcarriers being independently modulated with e.g. QPSK, 16 QAM, 64 QAM or higher modulation order allowing higher data rate per subcarrier. The higher modulation orders however can only be used if the signal to noise ratio (SNR) at the receiver is high enough to allow the demodulation. The subcarrier frequencies can be allocated to a user channel on a short term basis and the modulation order per subcarrier has to be selected to define a transmission channel for each user.
In a terrestrial mobile environment with multipath radio channels there is the possibility of a very strong channel attenuation of single subcarriers. This means that some subcarriers allocated to the user may be useless because of fading. In order to compensate the fading of some subcarriers, a known method, which is disclosed in document “Improving performance of multi-user OFDM systems using bit-wise interleaver” (Electronic Letters, 13th Sep. 2001, vol. 37, No. 19) by Z. Wang and R. A. Stirling Gallacher, proposes the allocation of subcarriers to a user channel according to a frequency interleaving scheme, that is the frequencies for the subcarriers allocated to a user channel are picked far enough apart so that they can experience different attenuation. By following this method, the bits are distributed over these subcarriers and an averaging effect is achieved, so that on average the raw bit error rate is acceptably low and a Forward Error Correction (FEC) coding mechanism is able to correct the bit errors.
While this allocation method significantly reduces the problem of fading, it still allocates subcarriers which can be useless for downlink transmission with a mobile station. Moreover, subcarriers may be allocated to a specific user channel which experience severe attenuation for this user and are thus not useful for downlink transmission to him, but which could be perfectly adequate, because not faded, for transmission to another user. Still another disadvantage of frequency interleaving allocation is that channel coding for the resulting bit error rate needs a lot of redundancy which reduces the usable net bit rate that can be transmitted over the channel.